Mechanistic aspects of paraquat toxicity in E. coli. A spin trapping study

Mechanistic aspects of paraquat monocation radical (PQ^.+ and copper involvement in paraquat toxicity have been examined using E. coli B cells. Electron spin resonance (ESR) spectrometry combined with cell survival studies were used to explore the correlation between radical production and biological damage. The line broadening agent oxalato-chromiate (CrOx) was used to characterize the anoxic partition of PQ^.+ inside and outside the cell. In the presence of CrOx the ESR signal was totally eliminated, indicating that intracellular species were undetectable and that, contrary to previous reports, PQ^.+ exclusively accumulates outside the cell. The PQ^.+ radical does not react with H₂O₂ but disappears in the presence of H₂O₂ when catalytic traces of Cu(II) are present. Spin-trapping studies using DMPO showed that in aerobic environment paraquat-induced O₂^- radicals are detectable exclusively in the extracellular compartment. The correlation between PQ^.+ appearance and the biological damage is not simple. PQ^.+ non-toxically accumulates, in the absence of oxygen and either Cu(II) or H₂O₂. By contrast, with both H₂O₂ and Cu(II) the cells are rapidly killed but PQ^.+ was undetectable. These results reconfirm the key catalytic mediatory function of transition metals in paraquat toxicity.